1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and an image forming apparatus and process cartridge using the electrophotographic photoreceptor.
2. Description of the Background
Organic photoreceptors are more widely used than inorganic photoreceptors in electrophotography recently. This is because a wide variety of environmentally-friendly organic materials responsive to either visible light or infrared light are easily available at low cost. However, organic photoreceptors generally have poor mechanical durability. Therefore, organic photoreceptors are required to have better mechanical durability as well as a long lifespan.
In a typical electrophotographic image forming apparatus, a photoreceptor is charged by a charger (“charging process”), an electrostatic latent image is formed on the charged photoreceptor, the electrostatic latent image is developed into a toner image (“developing process”), the toner image is transferred onto a transfer material (“transfer process”), and residual toner particles remaining on the photoreceptor without being transferred are removed (“cleaning process”).
The surface of the photoreceptor is chemically and physically damaged by repeated exposure to the above processes of charging, developing, transfer, and cleaning. The deteriorated photoreceptor produces poor quality image. Thus, there have been various attempts to improve mechanical durability of photoreceptors by providing a protective layer on their surfaces.
For example, Japanese Patent Application Publication No. (hereinafter “JP-A”) 2002-139859 describes an electrophotographic photoreceptor having a protective layer dispersing a filler. JP-2001-125286-A and JP-2001-324857-A each describe a photoreceptor having a protective layer having a high hardness. JP-2003-098708-A also describes a photoreceptor having a high hardness. As another example, JP-05-181299-A, JP-2002-006526-A, and JP-2002-082465-A each propose a protective layer including a thermosetting resin. JP-2000-284514-A, JP-2000-284515-A, and JP-2001-194813-A each propose a protective layer including a siloxane resin having a charge transportable group. Japanese Patent No. (hereinafter “JP”) 3194392 proposes a charge transport layer obtained from a monomer having a C═C double bond, a charge transport material having a C═C double bond, and a binder resin. JP-2004-302451-A proposes a charge transport layer obtained by curing a tri- or more functional non-charge-transportable radical-polymerizable monomer with a monofunctional charge-transportable radical-polymerizable compound. JP-2005-99688-A proposes a protective layer obtained by curing a tri- or more functional non-charge-transportable radical-polymerizable monomer with a charge-transportable radical-polymerizable compound, and dispersing a filler.
However, the lifespan is not necessarily extended only by improving mechanical durability. To extend the lifespan, photoreceptors are further required not to be adhesive to foreign substances and to be able to transfer toner at high efficiency.
First, why photoreceptors are required not to be adhesive to foreign substances is described below. Even a photoreceptor having high mechanical durability may produce defective images after a long period of use due to adhesion of foreign substances (e.g., paper powder or external additives released from toner), which prevents the photoreceptor from being normally exposed to charging and light irradiation. On the other hand, a photoreceptor having poor mechanical durability is unlikely to produce defective image even when foreign substances are adhered, because the substances can be removed along abrasion of the outermost surface. But such abrasion may adversely affect the lifespan of the photoreceptor. Thus, photoreceptors are required not to be adhesive to foreign substances.
Next, why photoreceptors are required to be able to transfer toner at high efficiency is described below. A photoreceptor which can transfer toner at high efficiency do not waste toner. When a large amount of toner particles are remaining on a photoreceptor without being transferred onto a recording medium, the photoreceptor may be excessively stressed with the action of a cleaning member, resulting in a short lifespan of the photoreceptor. Thus, photoreceptors are required to be able to transfer toner at high efficiency.
When a photoreceptor is non-adhesive to foreign substances and able to transfer toner at high efficiency simultaneously, it may be said that the photoreceptor has repellency. Repellency is effectively given to a photoreceptor by reducing the outermost surface energy. The surface energy can be reduced by applying an external low-surface-energy material to the photoreceptor or including an internal low-surface-energy material in the photoreceptor. For example, zinc stearate is usable as the external low-surface-energy material. The external low-surface-energy material can be applied from an application mechanism provided around the photoreceptor. However, there is a possibility that the external low-surface-energy material is deteriorated by electric discharge and defective image is produced thereby. Moreover, provision of the application mechanism limits the layout of image forming parts and raises the cost. As an example of using an internal low-surface-energy material, JP-2007-178815-A describes a photoreceptor including a fluorine-substituted polysiloxane resin in a surface layer. It is known that siloxane bonds polarize to form hydrogen bonds. Therefore, such a photoreceptor gets more adhesive to toner and degrades its repellency under high-humidity conditions. Undesirably, such a photoreceptor should be constantly abraded so that the internal low-surface-energy material is constantly exposed at the surface of the photoreceptor, sacrificing mechanical durability.
It is important, but is difficult, to satisfy both mechanical durability and repellency. JP-2002-6526-A describes a photoreceptor having a protective layer including lubricative fine particles. JP-2008-139824-A describes a photoreceptor having a fluorine-based hardened surface protective layer. JP-2008-233893-A describes a photoreceptor having a fluorine-based cross-linked surface layer and a protective layer including lubricative fine particles. Fluorine-based materials are effective for reducing adhesive force between photoreceptor and toner. A hardened protective layer including a fluorine-based material is effective for both improving mechanical durability and reducing adhesive force between photoreceptor and toner. But it requires a substantial amount of fluorine-based materials to sufficiently reduce the adhesive force between photoreceptor and toner. A substantial amount of fluorine-based materials may undesirably increase bright section potential and degrade film strength.
JP-2003-302779-A describes a photoreceptor having a surface layer comprising a specific resin having norbornene rings and a charge transport material. Because the resin is not cross-linked, the surface layer has poor mechanical strength a short lifespan. It is still difficult to provide a photoreceptor having a good combination of mechanical durability and high repellency.